Android 17 suddenly became one of the strongest heroes in the world of Dragon Ball when he was brought back for Dragon Ball Super's Tournament of Power. Originally, Android 17 and his sister, Android 18, were incredibly powerful villains who fought the Z-Warriors in the Cell Saga. Android 17 was matched up with Piccolo (who he was unable to beat) and absorbed by Cell. After Cell was defeated and 17 was saved, he disappeared from the anime. He was later used as a villain in Dragon Ball GT, which is no longer canon.
Android 17 made a proper return in Dragon Ball Super when Goku had to find fighters for Universe 7's Tournament of Power team. After squaring off against Super Saiyan Blue Goku, Android 17 proved that he was a force to be reckoned with. After joining the team, Android 17 participated in fights with Universe 11's Top, warriors from Universe 2, and helped Goku and Frieza fight a severely weakened Jiren. Surprisingly, Android 17 outlasted every fighter and became the last man standing when Goku and Frieza sacrificed themselves to take down Jiren. Thanks to 17's resilience, Universe 7 won the Tournament of Power.
Dragon Ball Super made Android 17 one of the anime's four strongest heroes, the others being Goku, Vegeta, and Gohan. But how did 17 suddenly become so powerful - let alone the breakout star of the Tournament of Power? 17 and his sister, Android 18, had comparable power levels in the Cell Saga, but 17 has surpassed her by a wide margin. 18 is nowhere near his level anymore and is likely closer to Piccolo in terms of strength, whereas as Android 17 is only slightly weaker than Gohan. So how did 17 get so far ahead? First of all, the staggering difference in power levels between 17 and 18 can be explained by looking at what the two have been doing since the Cell Saga ended.
Both androids now have families, but 17 has spent most of his time protecting his island from poachers. 17 has remained active and hasn't slacked off on his training, whereas 18 is less interested in fighting and more focused on being a full-time mother. So it makes sense that 17 would continue to improve while 18's power has remained somewhat consistent with what it was before. Also, the manga has revealed that 17 lives on his island with Cell Juniors, so he may have had training partners to keep him in shape.
It's important to note that 17 (and 18) have advantages that other characters in the Dragon Ball universe don't have. It was frequently pointed out in the Tournament of Power that the androids have infinite energy, which means that they never get tired. Not having a limit placed on much they can train is obviously a huge perk. This of course makes it easier for Android 17 to make gains than it would be for the other heroes, who don't share the benefits of infinite energy. Due to 17's physiology and his commitment to his training, he was able to keep up with the Z-Warriors off-screen and save the day for Universe 7 in Dragon Ball Super.